Derivation and assessment of a sex-specific fetal growth standard:
secondary analysis of a prospective observational study
Abstract
Objective: To derive and test the implications of a sex-specific fetal
growth standard. Design: Secondary analysis of a prospective
observational cohort. Setting: Eight U.S. centers. Population or Sample:
Nulliparas followed longitudinally through pregnancy. A lower-risk
subgroup (exclusions: chronic hypertension, pre-gestational diabetes,
suspected aneuploidy, preterm delivery) was selected for fetal growth
equation derivation. Methods: Fetal weights at 14-20 weeks, 22-29 weeks,
and birth were used to derive a sex-specific fetal growth equation. We
compared rates of SGA and LGA by sex using the sex-specific and
sex-neutral (Hadlock) standards. Using the full unselected cohort, we
assessed outcomes and clinical management according to SGA and LGA
status. Main outcome measures: Proportion considered SGA and LGA;
obstetric interventions relevant to SGA and LGA. Results: We derived a
sex-specific equation using 7,280 infants. The sex-neutral standard
diagnosed SGA more often in female and LGA more often in male newborns.
The sex-specific standard resolved these disparities. Using the full
unselected cohort (N=8,339), newborns reclassified from SGA to AGA by
the sex-specific standard were more likely to be delivered for growth
restriction with comparable risk of morbidity compared to newborns
considered AGA by both methods. Newborns reclassified from AGA to LGA by
the sex-specific standard had higher rates of cesarean for arrest of
descent, cesarean for arrest of dilation, and shoulder dystocia than
newborns considered AGA by both methods. Conclusions: The sex-neutral
standard generates sex disparities in SGA and LGA at birth. A
sex-specific standard resolves these disparities and may improve growth
pathology risk stratification.